This invention relates to a method of detecting the status of a keyboard switch device including a plurality of row conductors, a plurality of column conductors, an the matrix of a plurality of switching elements provided at intersections of the row conductors and the column conductors and operable in response to keyboard operation.
As well known in the art, a keyboard switch device includes a plurality of row conductors, a plurality of column conductors, and the matrix of a plurality of switching elements provided at intersections of the row conductors and the column conductors and operable in response to keyboard operation. In the keyboard switch device of the type described, simultaneous closure of two or more switching elements occurs very often. Such overlapping of closed switching elements is referred to as rollover.
Heretofore, proposals have been made of four methods to deal with the simultaneous closure of the switching elements.
(1) No rollover method PA1 (2) Two-key rollover method PA1 (3) N-key rollover method (hardware type, N represents a positive integer) PA1 (4) Oak's N-key rollover method with detection of a phantom condition (software type)
In the no rollover method, only the first-closed switching element is recognized and provided as valid output data. Once a particular key is pressed, no other keys are recognized until the particular key has been released.
In the two-key rollover method, two keys simultaneously pressed are both recognized. However, if more than two keys are pressed at the same time, only the switching elements corresponding to the first-pressed two keys are recognized and provided as valid output data.
In the N-key rollover method, at most N keys may be pressed simultaneously and each key will be correctly recognized and provided as valid output data. A problem with the N-key rollover method is a so-called phantom closure situation which will shortly be called a phantom condition hereinafter. The phantom condition arises when three switching elements at the three corners of a rectangular pattern are closed in the matrix of the switching elements. In this event, the switching element at the remaining corner of the rectangular pattern is falsely indicated as a closed element although the corresponding key is never pressed actually. One solution to the above-mentioned problem is to use diodes each of which is connected in series to each switching element. Alternatively, Japanese Patent Publication No. 99220/1986 (Tokkai Syo 61-99220) discloses the use of resistors each of which is connected in series to each switching element to isolate the switching elements from one another.
The Oak's N-key rollover method with detection of a phantom condition is disclosed in U.S. Pat. No. 4,420,744. In this method, row conductors and column conductors are scanned to detect the status of each switching element. A result of scanning operation is memorized in an array in a memory unit. The array is checked to determine if any phantom condition exists. After any switching elements in a phantom condition are eliminated, closed switching elements are provided as output data.
In this method, determination of existence of a phantom condition is carried out in a manner which will now be described. A pair of rows in the array are selected. Comparison is made of the information in each column position of the rows to determine if there are two columns where each row indicates a closed switching element. The operation is repeated until all combinations of rows have been selected.
In the above-mentioned methods (1) and (2), the number of the valid data is only one or two. Accordingly, use is restricted to the particular applications in which strict reliability is required.
The method (3) increases the cost of the device because the diodes or the resistors are required. In the latter, a level detection circuit is also necessary in addition to the resistors.
In the method (4), determination of existence of a phantom condition is very much complicated. Furthermore, the output data should be provided after eliminating any switching elements in a phantom condition.